1. Field of the Invention
The present invention is directed to a voltage regulating system for an electric generator and more particularly, to a voltage regulating system for an electric generator which suppresses an increase in field current when the number of revolutions of the generator is small such as when the generator is overloaded or during initial start up.
2. Description of the Prior Art
In an electric generator driven by an internal combustion engine, the output voltage is usually stabilized by feedback control of the field current supplied to the field coil of the generator, the feedback control being based on the output voltage of the generator. Thus, output voltage can be maintained at a prescribed level by allowing the field current to increase as the load increases. In an automatic voltage regulator of this type, however, if the generator is operated at low speed to avoid wasteful power operation of the engine as a driving source during a period of small loading or unloading, for instance, the automatic votage regulating function still operates and the field current supplied to the fiedl coil will unnecessarily increase. As a result, this not only causes the load for the field coil to increase but also increase the torque of the generator, and thus, the torque load of the engine as a driving source is uselessly increased.
As shown in FIG. 1, an automatic voltage regulator has been known which operates to supply the output current of an exciting coil 1 of a genrator to a field coil 3 through a control circuit 2. In this automatic voltage regulator, the output voltage of the generator is detected as a pulse voltage by a detection circuit 9 comprising a detection coil 4, a fullwave rectifier 5, a capacitor 6 and resistors 7 and 8. The voltage Vc obtained by dividing the pulse voltage with the resistors 7, 8 is compared with the Zener voltage of a Zener diode 10. A transistor Q1 in a control circuit 2 is turned OFF when Vc&lt;Vz and a transistor Q2 connected thereto, to form a Darlingtion pair is turned ON, so that the output current of the exciting coil 1 is supplied to the field coil 3 as a field current "if".
In other words, the field current i.sub.f is controlled by comparing the voltage Vc obtained by dividing the pulse voltage in the detection circuit 9, which varies upwardly and downwardly according to the variation of the output voltage of the generator with respect to the Zener voltage Vz. When a load connected to the generator is small, the driving torque of the generator is small because, as shown in FIG. 2(a), the section Vc&lt;Vz is narrow and the field current if supplied to the field coil 3 is small. However, the driving torque of the generator becomes large as the number of revolutions decreases with the load being increased, because the field current if supplied to the field coil 3 is increased, since the level of Vc drops as shown in FIG. 2(b), thus, making the section Vc&lt;Vz wider.
Accordingly, in this control system, it is necessary to enlarge the strength of recovery by increasing the energizing ratio t2/t1 of the field current which is a function of eo, the range of variation of Vc, so as to reduce the voltage regulation of the output voltage of the generator.
However, the arrangement shown in FIG. 1 makes the driving torque of the generator extremely large because a large field current if is being supplied to the field coil 3 until the generator gains (or recovers) the number of revolutions within normal operation when the generator is operated at relatively low speed as in an overload condition or during a starting operation. Consequently, a driving source with an output large enough to provide this large torque load value, or an engine with a large output is required for an engine-driven generator.
To lower the output of a driving source, it will be necessary to increase the voltage regulation by making a smaller energizing ratio t2/t1 for the field current which is a function of eo, the range of variation of the voltage Vc.